Many industrial processes require the conveying, delivering, pouring and dispensing of bulk powders. A number of properties of the powder affect its ease or difficulty of handling and may include electrostatic charge, specific gravity, particle size, particle shape and particle size and shape distributions. Other factors which may affect the handling properties of the powder are the presence of residual amounts of synthetic precursors, particularly solvents, the hygroscopicity of the powder and its degree of aggregation.
While it may be theoretically possible to measure all of the parameters quantitatively and then predict how the powder would handle, this rarely happens. In practice, a powder is matched with an appropriate piece of handling equipment using the long and short term experience of the designers of that particular system.
It is quite likely that new powder types or batches could be put into an existing system and new or unexpected problems would occur. A powder can form `bridges` and `rat holes` in delivery hoppers, build up on sloping surfaces to form unacceptable mounds, stick to walls and billow in clouds into the working environment. All these problems are related in some way to the physical properties mentioned above.
As the properties of a powder are so difficult to quantify, it is probable that the only way to check if a new powder type or batch will handle satisfactorily in an existing system is to try it out in that system. This means that a significant quantity of the material, commonly several kilograms, is required with the doubt that the material may not be acceptable and therefore have to be rejected.
When it is intended to produce an improvement in the synthesis of preparation of a powder, it is commonly more convenient, and much less expensive, to produce initially just a few grams which can be tested for its handling properties.
GB-B-2 060 902 discloses an arrangement and method for measuring the flowability of powders. The arrangement comprises a cylinder into which the powder to be tested is loaded (called the fill), and a series of disks each having a different sized hole drilled therein, the disks being attached to the undersurface of the cylinder and through which the powder is caused to pass. The arrangement operates on the principle that a free flowing powder slowly flows even through small holes until an inverted cone is formed in the whole thickness of the fill while a powder which flocculates into a mass falls as a lump leaving a cylindrical cavity in the fill. A positive result is obtained if the flow of powder begins within 60s and continues until a cavity of conical or cylindrical shape is formed in the whole thickness of the fill. If the test is positive, a smaller-sized hole will be used. Similarly, if the test is negative, a larger-sized hole is used.
The method described in GB-B-2 060 902 is time dependent, each run lasting at least 60s, and is repetitive regardless of whether a positive result is obtained during the first run. This method still needs a significant amount of powder and may still not predict how the powder falls past or builds up on a given surface.